1. Field of the Invention
Embodiments of this invention generally relate to a composite material having boron carbide, silicon carbide, and silicon as main components and to a method for manufacturing the composite material, and particularly relate to the composite material that has high specific rigidity and that can be manufactured at low cost and to the method for manufacturing the composite material.
2. Description of the Related Art
The specific rigidity is a parameter that is Young's modulus divided by specific gravity (ratio by weight for water), and a material which has high specific rigidity is occasionally required for various machine components. The examples thereof include, a three-dimension-measuring device and a linearity-measuring device which are devices with movable body requiring positioning functions of high accuracy, and an exposure apparatus for forming a pattern on a planar body. In particular, the exposure apparatus to manufacture a semiconductor wafer or a liquid crystal panel or the like has been required to have the positioning function of further higher accuracy satisfying the requirement of miniaturization of the pattern in recent years, and it has also been required to improve its through-put by moving at high speed a movable body such as a static pressure fluid bearing device on which a work to be exposed or a reticle is mounted, for economically forming a pattern.
However, to move such a movable body at high speed is necessarily to generate vibration, and this is a negative factor for the positioning accuracy. For quickly attenuating the vibration, a material with high specific rigidity is required, and for moving the movable body at high speed under a constant driving force, weight saving of movable parts is required, and bending of the apparatus leads to lowering of the positioning accuracy, and also therefore, a material with large Young's modulus and small specific gravity has been required.
As such machine parts requiring high specific rigidity, conventionally, metal materials such as iron and steel have been used. However, recently, alumina ceramics with higher specific rigidity has been used. However, in the case that further higher specific rigidity is required, it is necessary to use not oxide ceramics such as alumina but non-oxide ceramics. And among them, a boron-carbide-based material having the maximum specific rigidity as an industrial material is being expected.
As the boron-carbide-based material, the highest specific rigidity is expected in an approximately pure boron carbide sintered body, but boron carbide is known as a material difficult to be sintered. Accordingly, a conventional boron carbide sintered body has been manufactured by hot pressing. However, in the hot pressing sintering method, it is difficult to manufacture a product with large size and complex shape, and moreover, cost of the hot pressing apparatus or mold for providing high temperature and high pressure is large and therefore the method cannot be a method for realistically manufacturing the structural members.
For solving this problem, a technique of slip casting and pressureless sintering of boron carbide has been disclosed (see, for example, Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6). However, in this method, because the sintered body has difficult grindability, there are problems that grinding cost is larger for in use application requiring high accuracy of size such as semiconductor and liquid crystal manufacturing apparatuses, and that sintering cost is larger because the pressureless sintering temperature is 2200° C. or more, which is considerably high.
Accordingly, there has been disclosed a material in which the boron carbide is not sintered but a boron carbide powder is dispersed as a filler in a metal matrix phase (see, for example, Patent Document 7). In this material, boron carbide is dispersed in aluminum. However, because wettability between boron carbide and aluminum is bad, it is manufactured by hot pressing the mixture of boron carbide and aluminum, and in hot pressing, a product with large size and complex shape cannot be produced and the manufacturing cost is large, and therefore, the method cannot be a method for realistically manufacturing the structural members.
Accordingly, there have been disclosed composite materials each in which silicon whose wettability with boron carbide is relatively excellent is used as a metal matrix and the melted silicon is impregnated into the boron carbide molded body (see, for example, Patent Document 8, Patent Document 9, Patent Document 10). Among them, there is an example including a raw material that can be a small amount of carbon source as the primary material. However, in this method, because boron carbide is highly filled in the composite material although silicon is impregnated, the difficult grindability is not changed although the grindability is improved slightly more than that of the boron carbide. Moreover, because silicon is filled in the gap of the molded body having boron carbide as the main component, the completed composite material comes to contain a large amount of silicon, and such material has a low specific rigidity, and. the high specific rigidity of boron carbide cannot be applied.
Moreover, there have been disclosed composite material: each in which silicon carbide in addition to boron carbide is contained as a raw material of the molded body, and melted silicon is impregnated into the molded body (see, for example Patent Document 11). Among them, there is an example including a raw material that can be a small amount of carbon source as the primary material. However, in this method, all the same, because boron carbide and silicon carbide are highly filled in the composite material, the difficult grindability is not changed although the grindability is improved slightly more than that of the boron carbide. Moreover, because silicon is filled in the gap of the molded body having boron carbide and silicon carbide as the main components, the completed composite material comes to contain a large amount of silicon and such material has a low specific rigidity, and the high specific rigidity of boron carbide cannot be applied.                Patent Document: International publication WO 01/72659A1 pamphlet (Page 15-16)        Patent Document: JP-A 2001-342069 (Kokai) (Page 3-4)        Patent Document: JP-A 2002-160975 (Kokai) (Page 4-6)        Patent Document: JP-A 2002-167278 (Kokai) (Page 46)        Patent Document: JP-A 2003-109892 (Kokai) (Page 3-5)        Patent Document: JP-A 2003-201178 (Kokai) (Page 4-9)        Patent Document: U.S. Pat. No. 4,104,062 specification (col 2-5)        Patent Document: U.S. Pat. No. 3,725,015 specification (col 2-6        Patent Document: U.S. Pat. No. 3,796,564 specification (col 2-13)        Patent Document: U.S. Pat. No. 3,857,744 specification (col 1-3)        Patent Document: JP-A 2007-51384 (Kohyo) specificatioin (page 20-22).        